The present invention relates to the structure of electrodes for semiconductor devices.
In prior art semiconductor devices, the electrode structure has been designed and determined by considering the behavior of majority carriers, and not of minority carriers, which have a large influence on the characteristics of semiconductor devices. For instance, in the prior art P-N junction type semiconductor diodes (FIG. 1), the diode comprises a P.sup.+ semiconductor layer 11 with a high impurity concentration, a P.sup.- semiconductor region 12 having a low impurity concentration, an N.sup.+ semiconductor layer 13 containing N-type impurities at a high concentration value, and metal electrodes 14 and 15 which are formed to sandwich the structure constituted with these layers and a region.
When a forward voltage is applied to the electrodes, majority carriers (holes in this case) can pass through said P.sup.- layer 12, P.sup.+ layer 11 and the metal electrode 14 by means of their free movement within the valence band. This is illustrated in FIG. 2. Minority carriers (electrons in this case), however, which are injected from the N.sup.+ layer 13 into the P.sup.- layer 12 and move from the P.sup.- layer 12 to the metal electrode 14, are usually reflected and accumulated by a potential barrier .phi. of a high-low junction created between the P.sup.- layer 12 and P.sup.+ layer 11, so that a current conduction is restricted in the prior art P-N diodes, resulting to a large forward voltage drop. Another disadvantage of the prior art P-N diodes is a low speed recovery, resulting from the minority carrier accumulation by the high-low junction. These phenomena have been already reported in the technical article entitled "On Carrier Accumulation, And The Properties Of Certain Semiconductor Junctions" written by J. B. Gunn for J. Electron Contr. (Vol. 4, pp 17-50, 1958).
In order to solve the problem above mentioned, it may be proposed to remove the P.sup.+ region 11. It is not practical, however, because the P.sup.- region 12 is required to have a low impurity concentration less than 10.sup.17 /cm.sup.3 in order to obtain a sufficient reverse blocking voltage. In such a low impurity concentration, an extremely high contact resistance appears between the semiconductor and the metal electrode, resulting to a large forward voltage drop. In order to obviate such difficulty of the contact resistance, the P.sup.+ region 11 having the impurity concentration more than 1.times.10.sup.18 /cm.sup.3 would be required as taught in the prior art.
For these reasons, the prior art semiconductor diode has a relatively large forward voltage drop, which results in a large power dissipation or loss. Further, minority carriers are accumulated as mentioned above in the semiconductor layer 12, thereby a reverse recovery time constant being enlarged and a reverse recovery being delayed.
Difficulty like the above is concerned not only with the electrode structure of the diodes, but also with that of other semiconductor devices such as thyristors, transistors and so forth.